GAS GLANDS OF SOMK TELEOSTEAN FISHES. 185 



forms, viz. tliat of the Common Eel, Angailla valyarls. The " red 

 body " of the Eel has been described several times previously, 

 e. g. by Jacobs (42, 43), Quekett (61), and Corning (38), but never, 

 I ve]iture to think, in a sufficiently clear or comprehensive 

 manner and certainly never adequately illusti'ated. 



The Vascwlo-glcmdalar Structure hi the Bladder of the Common Eel 

 (Anguilla vulgaris). 



It is well known that the bladder of the Eel is a.n elongated 

 sac lying dorsally and towards the posterior end of the body 

 cavity and connected with and opening into the oesophagus by 

 means of a long wide duct — the ductus pneumaticus (text-fig. 53, 

 P.D., p. 186). The pneumatic duct opens into the bladder at about 

 midway in its length, the bladder in consequence being divisible 

 into anterior (A.B.) and posterior (P.B.) regions. The bladder 

 and duct are both situated in the same horizontal plane, ?'. e. the 

 duct opens into the bladder at the side, not dorsally or ventrally. 

 Running along the vential side of the duct are an artery and 

 a vein situated close together (A.Y.). This artery and vein give 

 rise to the two ovoid retia mirabilia (R.M.) where the duct opens 

 into the bladder, the two retia lying one on each side of the 

 duct and therefore being dorsal and ventral in position respec- 

 tively and in the same vertical line. 



The wall of the bladder (see PI. II. figs. 1-6, e.g.) in fish generally 

 is best described as consisting of the tunica externa and tunica 

 interna. The former consists of two definite layers of fibrous tissue, 

 the outer being loose in texture, often containing muscular tissue 

 and varying greatly in thickness, the inner dense, silvery in appear- 

 ance, aiid principally consisting of elastic fibres. The latter consists 

 of a thin basis of dense connective tissue, with wluch is closely 

 connected the innermost layer of the bladder wall, a layer con- 

 sisting usually of flattened cells. It is this innermost cellular layer 

 which gives rise to the mass of glandular cells in connection 

 with a rete mirabile — the gas gland. The tunica interna is 

 easily separable from the tunica externa, and it is advisable to 

 remove this latter before cutting sections of the gland. 



The vascular supplj' of the bladder may first be considered. It 

 has already been mentioned that an artery and a vein run along 

 the venti'al side of the wall of the ductus pneumaticus. These 

 two vessels are situated in the outer fibrous layer of the tunica 

 externa (PI. II. fig. 1), which is here of considerable thickness, and 

 they supply several arteries to the duct and receive several veins 

 from it. Shortly liefore these two vessels reach the region of the 

 jimction of the duct with the bladder, the artery and the vein 

 each divide into two (fig. 2), so that there are two pairs of vessels 

 instead of one, each pair consisting of an arteiy apd a vein. 

 ISTow the artery and vein of each of the.se pairs undergoes further 

 subdivision, the smaller arteries and veins resulting fi'oni this 

 subdivision intermingling in their course, but not once' com- 

 municating \\\\X\ each other, so that we now have situated towards 



